This invention lies in the field of polymer chemistry in which the polymers formed are suitable for coating biosensors. The coatings act to decrease the impedance at the sensor""s electrode and thereby enhance the signal during in vivo placement of the sensor.
Biosensors are small devices that use biological recognition properties for selective analysis of various analytes or biomolecules. Typically, the sensor will produce a signal that is quantitatively related to the concentration of the analyte. To achieve a quantitative signal, a recognition molecule or combination of molecules is often immobilized at a suitable transducer which converts the biological recognition event into a quantitative response.
A variety of biosensors have been developed for use with numerous analytes. Electroenzymatic biosensors use enzymes to convert a concentration of analyte to an electrical signal. Immunological biosensors rely on molecular recognition of an analyte by, for example, antibodies. Chemoreceptor biosensors use chemoreceptor arrays such as those of the olfactory system or nerve fibers from the antennules of the blue crab Callinectes sapidus to detect the presence of amino acids in concentrations as low as 10xe2x88x929 M. For a review of some of the operating principles of biosensors, see Bergveld, et al., ADVANCES IN BIOSENSORS, Supplement 1, p. 31-91, Turner ed., and Collison, et al., Anal. Chem. 62:425-437 (1990).
Regardless of the type of biosensor, each must possess certain properties to function in vivo and provide an adequate signal. First, the elements of the biosensor must be compatible with the tissue to which it is attached and be adequately shielded from adjacent tissues such that allergic or toxic effects are not exerted. Further, the sensor should be shielded from the environment to control drift in the generated signal. Finally, the sensor should accurately measure the analyte in the presence of proteins, electrolytes and medications which may interfere.
One of the problems with implantable biosensors occurs as a result of xe2x80x9croad blockxe2x80x9d type interference. This problem is encountered when the outermost layer of the biosensor has some hydrophobic characteristics. These characteristics result in the accumulation of plasma proteins on the surface of the electrode after only short periods of direct contact with body fluids. The hydrophobic regions of the sensor surface are believed to denature the proteins resulting in large deposits of protein mass. The deposits then affect the sensor""s performance through a physical interference in a xe2x80x9croad blockxe2x80x9d type of effect. The protein deposition is a gradual process which creates a non-uniform, non-predictable diffusion path for the analyte to the sensor. Moreover, the effect on the sensor is a cascading type in which the protein deposits dissapate the normal voltages applied to the electrodes (i.e., the deposits increase the capacitance of the system). The resultant requirement for higher voltages to offset the increased capacitance increases the noise, ultimately compromising the validity of the sensor""s output.
Other problems are also associated with implantable sensors having hydrophobic regions at the sensor""s surface. In particular, subcutaneous tissue contains substantial amounts of lipid vesicles. By implanting a biosensor directly into tissue, a portion of the sensor may be implanted directly into, or flush against a very hydrophobic lipid region. This also limits the aqueous environment which is required around the sensor""s electrodes.
What is needed in the art are new coatings for implantable sensors which are extremely hydrophilic and provide a substantial and uniform aqueous flow around the sensors. Quite surprisingly, the present invention provides such coatings and sensors equipped with those coatings.
The present invention provides methods for reducing the electrode impedance of implantable biosensors by coating the surface of the biosensor with a uniform hydrogel which allows unimpeded water movement around the sensor. The surface coatings are compositions which are biocompatible and are capable of water uptake of at least 120% of their weight, more preferably at least 200% of their weight. Upon the uptake of water, the hydrogels used in the present invention will also swell and provide a layer of water around the electrodes to which the hydrogels are attached.
In one group of embodiments, the hydrogels can be prepared from:
(a) a diisocyanate,
(b) a hydrophilic polymer which is a hydrophilic diol, a hydrophilic diamine, or a combination thereof, and optionally,
(c) a chain extender.
The present invention further provides implantable biosensors for measuring a variety of analytes, the biosensor having a coating as described above.